1895.] Br Gaskell, Tlie Origin of Vertebrates. 35 



find in the highest forms a great length of intestine innervated by 

 the vagus nerve, between the hind-gut with its innervation from 

 the sacral region and the fore-gut innervated by the glosso- 

 pharyngeal and facial as well as by the vagus. 



Passing downwards we see the fore-gut becoming branchial still 

 connected with the hind-gut by a long intestine innervated by the 

 vagus nerve, but the length of this latter in proportion to the 

 length of the whole is much less. In fishes the length becomes 

 smaller and smaller, the tube is straight instead of thrown into 

 innumerable folds and at last in the Ammoccetes we find a small 

 intestine innervated by the vagus between the large branchial 

 fore-gut and smaller hind-gut. In other words, the phylogenetic 

 history of the vertebrate gut shows that the line of tendency as we 

 descend the phylum is steadily towards an approximation of the 

 cranial and sacral portions of the gut: pass one step further and we 

 should find as in Limulus the end of the branchial chamber close 

 to the cloacal termination of the gut, i.e. close to the cloacal 

 termination of the old gut. 



In other words, the branchial chamber was continued on as a 

 short tube which broke through into the old existing cloaca and 

 the elongation of that tube is the striking fact in the phylogeny 

 of the vertebrate gut. The formation of this tube from the 

 respiratory region by the coming together of the ciliated hyper- 

 and hypo-pharyngeal ridges is well seen in Ammoccetes, and 

 embryologically the travelling down of the vagus and the fact 

 of this part of the gut being always supplied by the great branchial 

 nerve, the vagus, is strong confirmatory evidence that the middle 

 part of the intestine has arisen in the vertebrate phylum as the 

 gradual elongation of an originally small junction between the 

 respiratory chamber and the cloacal region. The evidence of this 

 junction taking place is preserved for us embryologically by the 

 breaking through of the septum of the proctodeum just as the 

 passage from the mouth region to the respiratory chamber through 

 the operculum is represented by the breaking through of the 

 septum of the stomatodaeum. 



Again, the embryological development of the Vertebrate illus- 

 trates this history in the most striking way, for not only does it 

 show that the embryo grows in length by additions made between 

 the head end and the tail end, but also that at a time when the 

 fore-gut and hind-gut are well formed the mid-gut is still a mass of 

 yolk ; this is especially well shown in Shipley's specimens and is a 

 clear indication that the formation of the mid-gut takes place em- 

 bryologically and therefore took place phylogenetically much later 

 than the formation of the fore-gut or hind-gut, for the argument 

 often used, that yolk retards development and for that reason the 

 mid-gut is formed latest, is to my mind evasive rather than real, 



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